Digital images are used in diagnostic radiology. The digital images are derived numerically. A standard analog signal is converted to digital values for computer purposes. After the computer has performed its function, the results are re-converted into an analog image for analysis. The analog image is displayed on a CRT or video monitor. The image is black and white with intermediate shades of grey. It is known that the human eye can only distinquish approximately twenty shades from white to black with intermediate greys.
The image on the monitor is made up of pixels. Each pixel is electronically assigned in brightness range from black to white. The combination of the pixels makes the analog image. Radiologists are trained to interpret an analog image using a greyscale. The color contrasts of an analog image displayed on a monitor are somewhat confusing. A natural progression is perceived by the eye from blackgrey-white. The human perception of about twenty grey levels is utilized to build the analog image. Individual numbers are assigned to a pixel making up the image projected on the monitor. Each pixel is assigned a level of grey depending on the numerical value that the pixel represents. Pixels are arranged in a matrix.
The numerical values in the digitized information is substantially greater than the number of grey levels that can be perceived with the human eye. The distribution of the greyscale relative to the digital values can be assigned to maximize the limit of the human perceived greyscale. This is accomplished by using window and level values. The entire range of digitized information is displayed by dividing the range of information into twenty shades of grey. Each shade of grey is assigned to a subgroup of twenty subgroups of digitized numerical values. Using the values from a modern computer tomography (CT) unit, as an example, the digitized values from -1,000 to +3,000 are available. The total range is 4,000 CT numbers. Hence, 200 digitized numbers would be assigned to each shade of grey. Black would be used for values -1,000 to -800. The next blackest grey shade would be used for those pixels with a numerical value from -800 to -600. This arrangement is repeated until a white would be used for pixels ranging from +2800 to +3,000. The differences of digitized values would not be seen in the analog image unless the pixel values change by 200 numbers thereby causing a change in the grey. Different and smaller increments in the change of digital values can be displayed on a modified greyscale.
The middle shade of the greyscale is called the level or center. The middle greyscale shade can be assigned to any digitized value. The range of digitized value that the greyscale spans is called the window. The greyscale is distributed symmetrically. Ten shades of grey are on each side of the level and each of the shades represents an equal range of digitized values.
The greyscale need not be spread out to cover all digitized values. Greyscale displayed could be distributed over a very limited portion of the digitized information. As an example, with digital values from -1,000 to +3,000, the interest in a narrow range of pixel values such as -110 to +190 one could assign the middle shade of grey to +40 and spread the twenty shades over a range of only 300 numbers. In this example, the range of digital values, 300 over which the greyscale is distributed is called the window. The level is +40. Ten shades of grey lie above +40 and ten shades of grey lie below +40. If the digital pixel value lies above the window, outside of the assignment of the greyscale, the pixel will be white. If the digital pixel values lies below the window, the pixel will be black.
Each of the twenty grey shades will represent 15 digitized computed tomography numbers (CT numbers) using the above windows. Only a change of 16 numbers, at most, is required before a change in the pixel greyness is detected. With this window, smaller changes in CT number will be detected than in a window, smaller changes in CT number will be detected than in a window of 4,000 where a change of 200 is needed before noting a change in greyness of a pixel.
If the interest is narrowed to only a very small subsegment of a digital information, for example, 80 numbers, the window would be 80. If those numbers range from 0 to +80, as in a head computed tomography scan, then the window would be assigned at +40. Therefore, the greyscale would be centered at +40 and would cover a numerical CT number values from 0 to +80. CT number values below zero would be black and CT number values above +80 white.